Micro-electromechanical system (MEMS) mirrors (or micro-optical mirrors) are essential to integration of optical and electronic systems including scanners, optical switches, and display systems. The micro-optical mirror structure generally includes moveable mirrors fabricated by micro-electronic processing techniques on wafer substrates including glass and semiconducting substrates using techniques that are similar to manufacturing techniques for manufacturing integrated circuits. For example, a moveable micro-mirror includes a mirror and support structure suspended over a conducting substrate. In one approach, the mirror is moved with respect to a conducting substrate by an electric field formed between the mirror and the surface of the conducting substrate. In other approaches magnetic materials and magnetic fields are used to move the mirrors.
Generally, prior art fabrication methods have included forming a conductive mirror support in contact with integrated circuitry formed in a semiconducting substrate to control the application of electric or magnetic fields to move a mirror attached to the mirror support. Generally, two methods have been used in the prior art for forming a conductive mirror support and an overlying mirror assembly. One method includes patterned etching of a layer of conductive material formed over the semiconducting substrate to form the conductive mirror support. Another method includes forming a dielectric insulating layer over a layer of conductive material overlying the semiconducting substrate and etching patterned openings in the dielectric insulating layer to electrically communicate with a conductive layer the openings corresponding to a width or diameter dimension of the conductive mirror support. A metal layer is then selectively deposited to fill the openings and to extend above the level of the dielectric insulating layer to form a conductive mirror support member.
Mirror assemblies, according to the prior art have generally been created by etching a convex or concave pattern in a separate thin semiconducting substrate to form an array of mirrors which is then attached by bonding the mirror array over a substrate including the support members. Various patterning processes have be proposed including ion implantation from one side of a silicon substrate surface to produce an etch stop followed by etching both sides of the silicon substrate to form the mirror array assembly. The mirror surface is typically optimized for a particular application by selective metal deposition including chemical vapor deposition or electrodeposition processes. Another approach is to pattern and etch in a separate thin semiconducting substrate, for example polycrystalline silicon both the mirror support member and the mirror and bonding the substrate to a base substrate including integrated circuitry for producing electric or magnetic fields.
One problem with prior art processes is the numerous patterning and etching steps required to form a mirror assembly, also referred to as a MEMS mirror or a DMD (digital micro-mirror device). Another problem is consistent production of radii of curvature for the mirror surfaces, prior art process frequently introducing aspherical portions that lead to image distortion due to local variations in etching processes, including reactive ion etch (RIE) processes. In addition, it is frequently necessary to subject the substrate and mirror components to elevated temperatures during the processing steps which further introduce thermal expansion mismatches which may lead to stress buildup on mirror surfaces, further degrading optical quality.
There is therefore a need in the micro-optical mirror fabrication art to develop a micro-optical mirror fabrication method whereby a mirror surface may be produced with fewer processing steps to provide a more consistent radii of curvature.
It is therefore an object of the invention to provide a micro-optical mirror fabrication method whereby a mirror surface may be produced with fewer processing steps to provide a more consistent radii of curvature while overcoming other shortcomings of the prior art.